1. Field of the Invention
The present invention relates to a fail-safe control circuit for improving the controlling accuracy of a controlled device controlled by operation control units and, more particularly, the present invention is directed to a fail-safe control circuit which allows the operation control units to monitor a control signal for correctness and if any of the operation control units detect an error, the fail-safe control circuit fail-safes the controlled device by interrupting power thereto.
2. Description of the Related Art
A fail-safe control circuit is generally provided in control systems to force a controlled device into a safe state when a malfunction occurs and the proper operation of such a fail-safe control circuit is very important when it is concerned with the control of a device affecting people's lives. For example, in an anti-skid brake control circuit for a car, vehicle velocity and wheel rotation are monitored and supplied to an operation control unit such as a microcomputer. When wheel locking is detected, the braking operation is released to prevent the wheels from slipping thereby improving the braking function making it possible to accurately stop the car.
In an anti-skid brake control circuit, it is important to improve reliability, and therefore each control circuit is provided with a plurality of identical operation control units arranged in parallel to process the same input in synchronism with each other using the same clock signal. The operation control units control the controlled device using an output signal obtained based on majority voting type processing performed on the respective outputs of the operation control units to thereby improve reliability.
In the circuits arranged in the manner described above, an erroneous signal may be produced by the majority voting circuit when one half or more of the plurality of operation control units operate erroneously. As a result, it is impossible to determine whether the controlled device is operating correctly in accordance with the control output. In a case where, for example, a disconnection occurs in a solenoid which is the controlled device, the desired controlling operation cannot be performed which will cause a serious accident.